1. Field of Invention
This invention relates to an irregular-section tubular body and an axle beam for a torsion beam and a method of manufacturing the same. In particular, the manufacturing method of the present invention manufactures the irregular-section tubular body having a concave portion by press-forming a tubular worked body by applying liquid pressure to an inner surface thereof. The axle beam for a torsion beam of the present invention is joined, if the torsion beam is used in a rear-wheel suspension device of a vehicle, to a pair of trailing arms to attach wheel sides integrally to the axle beam.
2. Description of Related Art
A manufacturing method for an irregular-section tubular body having a concave portion, concaved in a direction perpendicular to axis thereof (hereinafter simply referred xe2x80x9caxis perpendicular directionxe2x80x9d), at least part of the concave portion being in an axial direction is known. One such manufacturing method includes press-forming a tubular worked body by applying pressure to a liquid sealed in an internal space of a tubular body to an inner surface thereof.
For example, Japanese Patent Laid-open (Koukai) No. 8-90097 discloses a manufacturing method for manufacturing an irregular-section tube having a shape of which varies in a longitudinal direction by usage of liquid pressure. In this manufacturing method, an internal space of a circular tube as the worked body is filled with liquid. Further liquid is then supplied by a pump to apply predetermined pressure P1 to an inner surface of the circular tube. In this condition, the circular tube is press-formed by a pair of upper and lower press molds of predetermined shape and subjected to bending work. Finally, further liquid is supplied into the circular tube to increase the liquid pressure to a predetermined value P2 so that the circular tube bulges or expands along mold surfaces of the upper and lower press molds. Thus, the circular tube is formed into the final irregular-section shape to complete the formation.
Using the above irregular-section tubular body, an axle beam formed and used for a torsion beam as a rear-wheel suspension device of a vehicle is also known.
For example, EP 0650860 B1 discloses, as shown in the attached FIG. 20, a torsion beam comprised of a pair of trailing arms 91 attached to wheel sides, and an axle beam 92 each end of which is joined to each of trailing arms 91, respectively, to connect them integrally to the axle beam. The axle beam 92 is formed into a shape to have a large bending rigidity and small twisting (torsion) rigidity. As shown in the attached FIG. 21, the axle beam 92 has cross-section in the axis perpendicular direction of closed section shape including a closed space therein. The cross-section has a star shape in which three wing portions 92a extend radially outwardly and equidistantly in a circumferential direction.
However, the conventional manufacturing method of the irregular-section tube body requires both the press-forming process which bends the circular tube by the press molds, and the liquid pressurizing process, which pressurizes liquid in the circular tube by the liquid pressurizing device to obtain the final irregular-section shape of the tubular body. For these reasons, conventional manufacturing methods have been suffering from the disadvantages of the manufacturing process being complex and troublesome.
In addition, since the circular tube is bulged only by the liquid pressure to follow to the mold surfaces, very large liquid pressure is needed for the bulging. For this reason, the liquid pressurizing device used to positively pressurize the liquid, and a large-size extra hydraulic pressing device used to apply a mold fastening force to the press molds against the increasing liquid pressure are necessary. As a result, not only does the equipment become complex and large-sized, but equipment cost increases.
In the above conventional axle beam 92, as shown in FIG. 21, three wing portion 92a of the same size and same shape and extending radially outwardly and equidistantly in the circumferential direction are disposed. As a result, it has a cross-section shape in which a centroid and a shearing center coincide. Such coincidence decreases the roll rigidity in the axle beam 92 and creates an over-steer tendency, whereby the steer stabilizing is decreased.
The present invention has been made in view of the above circumstances and provides a manufacturing method for an irregular-section tubular body which simplifies the manufacturing process and excludes the additional liquid pressurizing device and the extra hydraulic pressing device, thereby avoiding the complex and large-sized equipment and an increase equipment cost are avoided. Also, the present invention provides an axle beam for the torsion beam which provides an increase in the steer stabilizing character by an improvement of the section-shape in the axis perpendicular direction thereof. Further, the present invention provides upper and lower press molds for press-forming a tubular worked body to the axle beam for the torsion beam.
In a first embodiment of the manufacturing method for the irregular-section tubular body comprises a tubular worked body, with a sealed liquid therein, is press-formed by an upper press mold and a lower press mold at least one of which has at least one convex mold portion to form a concave portion concaved in an axis perpendicular (radial) direction at an axial part of the worked body by the convex mold portion; and the press-forming of the worked body to the final tubular body is completed by a one stroke process from a press start point to a press complete point by using a liquid pressure increasing as a decrease of the internal volume of the worked body occurs due to formation of the concave portion, without pressurizing the liquid in the worked body by a pressurizing device.
In the suitable mode, the one stroke process includes a pressure increase process in the former half of the stroke from the press start point to a predetermined point, the pressure increase process press-forming the worked body with the liquid pressure gradually increasing as a decrease of the internal volume of the worked body, due to formation of the concave portion to an inner peripheral surface of the worked body; and a non-pressure increase process in the latter half of stroke from the predetermined point to the press complete point, the non-pressure increase process, following the pressure increase process, press-forming the worked body without applying the increasing liquid pressure to the inner peripheral surface of the worked body.
Here, the press-forming performed without applying the increasing liquid pressure on the inner peripheral surface of the worked body includes a press-forming performed (i) with applying a liquid pressure maintained in a constant pressure on the inner peripheral surface of the worked body, (ii) with applying a gradually decreasing liquid pressure on the inner peripheral surface of the worked body, and (iii) with applying a gradually decreasing liquid pressure on the inner peripheral surface of the worked body from the start to midway through a non-pressure increase process, and without applying liquid pressure on the inner peripheral surface in a final stage of the non-pressure increase.
In the suitable mode, the mold surface of the upper press mold and a mold surface of the lower press mold do not restrain a circumferential part of an outer peripheral surface of the worked body to leave a non-restrain surface at a portion where the concave portion is being formed. Also, in the non-pressure increase process, the liquid pressure applied to the inner peripheral surface of the worked body is maintained at a constant pressure or is decreased, by a liquid-discharge controlling device discharging liquid from the worked body at the predetermined point. Further, in the pressure increase process, with the worked body being set above the lower press mold having the convex mold portion, the upper press mold is descended so that the concave portion is formed on the worked body by the convex portion of the lower press mold.
The second manufacturing method for manufacturing an irregular-section tubular body includes a step for sealing fluid in a tubular worked body and for attaching a fluid-discharge controlling device discharging the fluid so that fluid pressure does not go over a predetermined pressure to the worked body; and a press-forming step for press-forming the worked body to apply an acting force thereto, increasing pressure of the fluid near to the predetermined pressure by decreasing a sealed space of the worked body, and then press-forming the worked body with discharging the fluid therein by the fluid-discharge controlling device so that the pressure of the fluid does not reach the predetermined pressure.
In the suitable mode, the fluid-discharge controlling device can be a relief valve or an accumulator operating when the pressure of the fluid reaches near to the predetermined pressure. Also, the liquid-discharge controlling device can be a restricted portion having a predetermined open rate and constantly discharging the fluid.
The axle beam of the present invention has, at least at an axial part, a concave portion concaved in an axis perpendicular direction, an axis perpendicular section at the concave portion having a closed section shape including a closed and sealed space, and a shape a shearing center being offset from a centroid by a predetermined amount.
Here, the axis perpendicular direction shape at the concave portion having a xe2x80x9cclosed section shape including a closed and sealed spacexe2x80x9d means an outer peripheral wall of the axis beam does not have circumferential ends to form a ring. In other words, the outer peripheral wall of the axle beam is formed by an annular wall. The outer peripheral wall formed by the annular wall can form single closed and sealed space, or plural closed and sealed spaces separated by an overlap portion where opposing inner surfaces of the outer peripheral wall are overlapped.
In the preferred embodiment, a circumferential part of the worked body opposite the concave portion in the axis perpendicular direction is so shaped that the worked body has a substantially U-shape or V-shape axis perpendicular section at the concave portion.
In the preferred embodiment, a rough tube comprising a large-diameter axially intermediate portion, a pair of small-diameter axially end portions having smaller diameter than the large-diameter axially intermediate portion, and a pair of diameter change portions connecting each end of the large-diameter axially intermediate portion. Each of the small-diameter axially end portions are subjected to a bending work to form each of the trailing arms from each of the small-diameter axially end portions and to form the axle beam by at least the large-diameter axially intermediate portion. Here, the large-diameter axially intermediate portion, the diameter change portions and the small-diameter axially end portions are connected by a continuous surface having no radial step, due to an offset of the small-diameter axially end portions from the large-diameter axially intermediate portion in the same direction of the shearing center being offset from the centroid, at a circumferential part of the small-diameter axially end portions at the offset side.
In the preferred embodiment, each of the small-diameter axially end portions are bent in a direction orthogonal to the axial direction of the axle beam and orthogonal to a concave direction of the concave portion.
An upper press mold and a lower press mold for press-forming an axle beam for torsion beam by press-forming a tubular worked body with sealed liquid therein, includes one of the upper press mold and the lower press mold has at least one convex mold portion to form at least one concave portion concaved in an axis perpendicular direction at an axial part of the worked body and an axial length shorter than that length of the worked body. The other of the press molds has a concave mold surface having an axial length substantially equal to the length of the worked body.
In the suitable mode, the upper press mold or the lower press mold having the convex mold portion has width smaller than dimension of the worked body in the axis perpendicular direction.
The first manufacturing method for the present invention completes formation of the final irregular-section tubular body by only press-forming the worked body having sealed liquid therein, not requiring the liquid pressurizing process after the press-forming. As a result, the manufacturing process for the irregular-section tubular body becomes simple. Also, the liquid pressurizing device or the large-size extra hydraulic pressing device are not needed, thus avoiding complex and large-sized equipment and avoiding increase of the equipment cost.
In the manufacturing method of the present invention, none of a rise in the speed of the liquid pressure (pressure increase speed) in the worked body, a maintained time period of the max. pressure, nor the pressurized state of the liquid after the liquid pressure reaches max. pressure, affect the quality of the press-formed article.
During the pressure increase process in the former half of the stroke, the worked body is press-formed with the liquid pressure increasing as the internal volume of the worked body due decreases due to formation of the concave portion, being applied on the inner peripheral surface. By press-forming the tubular worked body filled with the liquid by the upper press mold and the lower press mold, a portion, or part of, the worked body pressed by the convex mold portion of the press mold plastically deforms to be concaved in the axis perpendicular direction.
During formation of the concave portion, the internal volume of the worked body decreases. The worked body is press-formed with the liquid gradually increasing as the internal space of the worked body decreases due to the forces being applied to the inner peripheral surface thereof. For this reason, in the pressure increase process, not only the worked body can be deformed or shaped to follow the mold surfaces of the press molds, but the worked body can be prevented from buckling. Thus, the worked body is smoothly formed.
In the non-pressure increase process in the latter half of the stroke, the worked body is press-formed without the liquid pressure increasing stage being applied to the inner peripheral surface thereof. That is, in the non-pressure increase process, the worked body is press-formed (i) with the liquid pressure being applied to the inner peripheral surface maintained at a constant level, (ii) with the liquid pressure being applied to the inner peripheral surface thereof gradually decreasing, or (iii) with the liquid pressure being applied to the inner peripheral surface thereof from start to midway of non-pressure increase process gradually decreasing, and without the liquid pressure being applied to the inner peripheral surface thereof in a final stage. By press-forming the worked body without applying the liquid pressure under the increasing stage to the inner peripheral surface thereof, the liquid pressure in the worked body is prevented from excessively increasing.
A one stroke process of the press-forming from the press start point to the press complete point can also be comprised of only a pressure-increase process. The process applies liquid pressure gradually increasing as decrease of the internal volume of the worked body due to formation of the concave portion to the inner peripheral surface thereof.
As the concave portion is formed at the axial part of the worked body, a single concave portion, two or more axially separated concave portions, or a long concave portion axially extending over the worked body, can be adopted. Plural concave portions separated in the axial direction can be disposed at the same position or at the different (separate) positions in the circumferential direction of the worked body. Also, plural concave portions disposed at one axis perpendicular section of the worked body can be separated in the circumferential direction. There is no limit or restriction in the shape and size of the concave portion. When plural concave portions are provided, they can have different shapes and sizes.
The convex mold portion is provided on at least one of the upper press mold and the lower press mold. Shape, size, number and location of the convex mold portion can be selectively determined to correspond to that of the concave portion of the irregular-section tubular body to be formed. The mold surfaces of the upper press mold and the lower press mold, other than the convex mold portion, can also be selectively determined to correspond to an outer surface shape of the irregular-section tubular body to be formed.
The worked body can be press-formed by restraining the whole outer peripheral surface at the portion where the concave portion is formed, by the mold surfaces of the upper press mold and the lower press mold. However, press-forming the worked body without restraining the circumferential part(s) of the above portion by the press molds is preferable. In this case, the circumferential part(s) of the portion forms the restrained surface(s) which is not restrained by the mold surfaces of the upper and lower molds.
By constructing the upper and lower molds in this way, the convex mold portion can be a convex partial mold having a width shorter than an axis perpendicular width of the worked body (diameter when the worked body is tubular). This reduces mold cost and a mold wear compared to the upper and lower press molds of sealed and closed type.
Even when the worked body is press-formed without restraining the surface(s), excessive deformation of the worked body at the non-restrained surface can be prevented by controlling the max. pressure of the liquid in the worked body. The max. pressure the liquid may reach during the press stroke, including the press complete point, can be set to be smaller than the predetermined value by, for example, removing an increase of the liquid pressure in the predetermined point of the press stroke.
Also, in the first and second manufacturing methods, the liquid pressure applied to the inner surface of the worked body, in the non-pressure increase process, is preferably maintained at a predetermined pressure or decreased by using a fluid-discharge or liquid-discharge controlling device that discharges the fluid or liquid from the worked body at the predetermined point. With such a liquid-discharge controlling device, the liquid pressure applied to the inner surface of the worked body can be easily and accurately controlled, so that any increase of the liquid pressure at the predetermined point can be easily and securely removed.
The liquid-discharge controlling device is, for example, comprised of a discharge tube and a relief value. The discharge tube is connected to a seal plug sealingly closing the end portion of the worked body and communicating with the internal space of the worked body. The relief valve is provided on the discharge valve and automatically opens when the liquid pressure in the worked body goes over the set pressure and automatically closes when it goes below the set pressure.
Another type of liquid-discharge controlling device can be comprised of an accumulator including a cylinder member, a connect tube and a gas supply means. The cylinder member has a closed operating chamber of a predetermined capacity in which a partition member is disposed reciprocately. The partition member liquid-sealingly partitions the closed chamber into a liquid chamber and a gas chamber. The connect tube is connected at one end thereof to a water-discharge hole of a second seal plug and is connected at other end thereof to the liquid chamber of the cylinder member, to connect the internal space of the worked body and the liquid chamber of the cylinder member. The gas supply means is connected to the gas chamber of the cylinder member to supply gas of a predetermined pressure thereto.
Still another type of the liquid-discharge controlling device can be comprised of a discharge tube and a restrict portion. The discharge tube is connected to a seal plug sealingly closing the end portion of the worked body and communicating with the internal space of the worked body. The restrict portion is provided on the seal plug or the discharge tube and has a section-area of a flow passage so that the liquid pressure in the worked body becomes the max. pressure at the predetermined point in the press stroke.
Here, the above irregular-section tubular body is used to construct the torsion beam by bending both end portions thereof in another step so that at least the large-diameter axially intermediate portion forms the concave portion of the axle beam, while at least the small-diameter axially end portions form the trailing arms.
The axle beam for the torsion beam of the present invention is joined to the paired trailing arms integrally attached to the wheel side. The axle beam can be formed integral with each of the trailing arms joined to each end of the axle beam. Alternatively, the axle beam can be formed independent from each of the trailing arms, and then connected to them by welding, for example.
The axis perpendicular section of the axle beam, at the concave portion has a closed section shape including the closed and sealed space, and has the shape the shearing center being offset from the centroid.
Here, the xe2x80x9ccentroidxe2x80x9d of the axis perpendicular section shape means a center of gravity of a plane figure thereof. The xe2x80x9cshear centerxe2x80x9d of the axis perpendicular section shape means a point in the section where a composed force of shearing forces acts so that the axle beam is subjected to simple bending without generating a twisted deformation, even when the twisting force, for example, resulting from the force applied to the trailing arms, is applied to the section in addition to the bending moment. The concept of xe2x80x9cshear centerxe2x80x9d is well known in the field of material mechanics. The centroid and the shear center can be determined unequivocally based on the axis perpendicular section shape by a predetermined geometrical calculation.
The concave portion of the predetermined shape formed at the axial part of the axle beam, and the offset of the shear center from the centroid, at the concave portion, not only increases the roll rigidity, but diminishes under-steer and increases the tracking character of the vehicle. Thus, steering stability increases.
Due to the cross-section shape of the concave portion, the torsion rigidity and roll rigidity of the axle beam can be adjusted by the changing size or dimension of the closed and sealed space. For this reason, no additional stabilizer for the roll control is required. Also, the axle beam, having the closed cross-section shape including the closed and sealed space, does not have any shear edge portion generating fatigue cracks, so that endurance character has increased.
Regarding the offset amount of the shear center from the centroid in the axis perpendicular section shape, the above advantages resulting from the offset can not be rendered when the offset is too small. Also, the tracking characteristics of the vehicle is affected badly when the offset is too large. For this reason, the offset amount of the shearing center should be determined in range so that both requirements are satisfied. Regarding the offset direction of the shear center from the centroid in the axis perpendicular section shape, the shear center is sufficiently disposed at a position at least to be above the centroid when the axle beam is mounted onto the vehicle. However, for effecting the steering stabilizing character resulting from the offset, the shearing center is preferably disposed at a position to be just above the centroid when the axle beam is mounted onto the vehicle.
Regarding the shape of the axis perpendicular section shape at the concave portion, various shapes can be adopted as long as it has closed section shape having the closed and sealed space and the shearing center is offset from the centroid. The axis perpendicular section shape can be formed into a substantially U-shape,V-shape, Y-shape or laid U-shape. Among them, the U-shape or V-shape is preferable. In such shapes of the axis perpendicular section shape, the size of the closed and sealed space and the offset amount of the shear center can be easily changed. As a result, the twist rigidity, roll rigidity of the axle beam, and steering character can be easily adjusted. In addition, the shear center can be effectively offset from the centroid in a limited space.
In addition, with adjustment of the twist rigidity and roll rigidity of the axle beam, the twist rigidity and roll rigidity become larger as the closed and sealed space becomes larger. For this reason, the size of the closed and sealed space is suitably determined corresponding to the required twist rigidity and roll rigidity. The closed and sealed space can be formed by single closed and sealed space enclosed by an outer peripheral wall of the axle beam, or plural closed and sealed spaces separated by an overlapped portion where opposing inner surfaces of the outer peripheral wall are abutted.
In view of certain restrictions such as setting space in the vehicle, the axle beam for torsion beam, the trailing arms preferably have small size as thin as possible in the range where it has the necessary rigidity. For satisfying such a requirement, an axle beam shown in FIG. 18 can be adopted. For manufacturing the axle beam, a rough tube 60 is prepared. This rough tube 60 is comprised of an large-diameter intermediate portion 61 located at an axially intermediate position of the worked body, small-diameter end portions 62 located at both end positions thereof and having smaller diameter than the large-diameter intermediate portion 61, and outer diameter gradually change portions 63 connecting each end of the large-diameter intermediate portion 61 and each of the small-diameter end portions 62 respectively. All of the large-diameter intermediate portion 61, small-diameter end portions 62 and outer diameter gradually change portions 63 are disposed to be coaxial. By bending the rough tube 60, the torsion beam in which, at the both ends of the axle beam having the concave portion of the predetermined shape, a pair of trailing arms are joined integrally is manufactured.
After forming the concave portion 61a on the large-diameter intermediate portion 61, the rough tube 60 is subjected to the bending work. Thus, each of the small diameter end portions 62 forms each of the trailing arms, and at least the large-diameter intermediate portion 61 and each of the outer diameter change portions 63 form the axle beam.
However, following problems arise. In the rough tube 60 having three portions 61, 62 and 63 all of which are coaxial but each of which has different diameter, each of the outer diameter gradually change portions 63 gradually decreases the outer diameter thereof from one end of the large-diameter intermediate portion 61 to one end of the small diameter end portions 62 equally in the circumferential direction. As a result, a radial step H having equal height in the circumferential direction is formed between the large-diameter intermediate portion 61 and each of the small-diameter end portions 62.
For this reason, as shown in FIG. 19, even after the concave portion 61a has been formed on the large-diameter intermediate portion 61, the radial step H remains at circumferential part between the large-diameter intermediate portion 61 and each of the small-diameter end portions 62. A compression force applied to the axle beam in an axial direction thereof may cause a buckle at a portion of the step H, so that rigidity of the axle beam against the axial input may decrease. If the steps H exist at the both end portions of the axle beam, rigidity of the axle beam against the input from the lateral directions of the vehicle may decrease.
To the contrary, a rough tube in which each of the small-diameter axially end portions is offset from the large-diameter axially intermediate portion in the same direction as the offset direction of the shear center from the centroid can be used. In this rough tube, at a circumferential part at a side of the small-diameter axially end portions are offset, the large-diameter axially intermediate portion, outer diameter gradually change portions and small-diameter end portions are connected by a continuous surface without a radial step. By using such rough tube for manufacturing the axle beam, the continuous surface remains at the circumferential part even after the concave portion has been formed. In this way, the above step H that decreases the rigidity of the axle beam against the input in the axial direction can be removed, whereby rigidity against the input of axial direction can be increased.